1. Field of the Invention
The present invention relates to joined carbon electrodes and to a method for joining carbon electrodes used for an electric arc furnace, such as a steelmaking furnace.
2. Description of the Prior Art
As is well known, carbon electrodes, hereinafter simply referred to as electrodes, are used for an electric arc furnace, such as a steelmaking furnace.
A prior art is explained with reference to FIG. 1. The first electrode 1 is disposed above the electric arc furnace and is used for smelting. When the first electrode 1 is consumed and thus shortened, the first and second electrodes 1 and 2 are joined as described hereinafter. The first and second electrodes 1 and 2 have circumferential surfaces 1a and 2a, respectively, and are brought into contact with one another at their respective end surfaces 1b and 2b in the joining step. In the joining step, a nipple 3 is screwed into the socket 4 of the second electrode 2. The second electrode 2 is then suspended by a crane (not shown), is disposed above the first electrode 1, and is turned, thereby screwing the socket 4 of the first electrode 1 onto the nipple 3. As a result, the first and second electrodes 1 and 2 are secured to one another via the nipple 3. The screwing is continued until the first and second electrodes 1 and 2 are brought into contact with one another via the end surfaces 1b and 2b. Screwing is conventionally determined as being completed when it becomes impossible to turn the second electrode 2 with a predetermined torque. Thus, the conventional method for determining the completion of screwing is a rather sensory one. Conventionally, the nipple 3 is screwed into the second electrode 2 at a steelmaking plant. Recently, electrodes equipped with a screwed nipple 3 which is not offset toward one of the sockets 4 have been supplied to steelmaking plants. In these electrodes, the nipple 3 is preliminarily screwed into the socket 4 at such a precise position that the largest diameter part of the nipple 3 is not offset toward one of the sockets 4 of the two electrodes 1 and 2.
Regardless of whether or not the nipple 3 is preliminarily screwed into the socket 4, the operation for screwing the nipple 3 into the socket 4 of the first electrode 1 is not easy since the screwing is carried out during the suspension of the second electrode 2 from a crane (not shown). That is, during the screwing of the nipple 3 into the socket 4 of the first electrode 1, the second electrode 2 suspended from the crane must be precisely positioned above the first electrode 1, which is usually installed at an electric smelting furnace, and, furthermore, a crane is not very well adapted for minute adjustment of the position of the second electrode. During the screwing of the nipple 3 and the minute adjustment of the position of the second electrode, a high load is frequently applied to the sockets 4 and the nipple 3. Particularly, the threads of the nipple 3, especially the top threads, and the sockets 4 are exposed to the high load and may be broken. If the threads are broken, fragments thereof remain on the bottom threads and impede normal joining of the nipple 3 and the socket 4, and, thus, determination of the completion of screwing according to the conventional method is inaccurate since the nipple 3 and the socket 4 are not completely joined to one another. If the degree of incomplete joining is great, a clearance can be detected at the electrode-joining end 5. Usually, such incomplete joining is not so great as to be discernible with the naked eye and hence is not detected. If smelting is carried out using joined electrodes which are incompletely joined, accidents such as breakage of the nipple 3 and the socket 4 and the dropping off of the first electrode 1 can occur.
It is known from Japanese Examined Utility Model Publication No. 44-21745 that in order to join the nipple fitted to one of the electrodes to the socket of the other electrode by turning the one electrode, the starting point for turning the one electrode is specified by a mark on the nipple and a mark on the other electrode. The marks are positioned so that at the beginning of screwing of the nipple into the socket the marks coincide with one another and the screw threads stably engage with the screw bottoms. The one electrode mentioned above is first turned to make its nipple mark coincide with the mark formed on the other electrode and the screwing of the nipple into the socket is then started. The joining method disclosed in Japanese Examined Utility Model Publication No. 44-21745 allows only easy starting of the screwing of the nipple into the socket.